thumper

why would you say only one if there are only 5-6in long?

Even with 55gallons you will not be able to keep 2 standard/common/sailfin plecos, you could go for something like a pair of Ancistrus (britle nosed plecos) or a Royal Pleco or one of the other 5-6" variety but only one

Other than that sounds good, plants are your friend they help you keep your tank under control and they are not half as hard to keep as many people would tell you. No Co2 goes in my tanks and my plants thrive.

that should be absolutely fine, the barbs may get a bit nippy but your tank is way big enough for them.

Add them slowly one group of 6 a week once they are settled in you will have space for a centerpiece fish as well say a dwarf gourami of some variety

What are you going to use on the bottom of your tank sand or gravel, and we can look at a clean up crew for down there as well?